The ability of a medium to refract light is its refractive index ("RI"). RI is the ratio of the velocity of light in a vacuum to the velocity of light in a medium. It is a physical property of the medium and is represented by a dimensionless integer "n". Differential refractometry is the art of measuring small differences in RI between a reference solution and a sample solution. The difference in RI is referred to as ".DELTA.n". .DELTA.n is measured in RI units ("RIU").
Differential refractometers in the prior art generally consist of a light emitting diode (LED), a flow cell containing a sample side and a reference side, and a dual element photodiode detector. As illustrated in FIG. 1, known differential refractometers utilizing a dual pass optics bench contain a mirror which reflects the light causing the light beam to pass through the flow cell twice before reaching the photodiode.
After the light beam passes through the flow cell the second and final time, it passes through an imaging lens and then falls upon the dual element photodiode. When the flow cell contains just solvent, the position of the light beam is centered on the elements of the photodiode as shown in FIG. 2a. This position of the beam creates a baseline signal. When sample is inserted into the flow-cell, light beam is refracted further, causing a deflected image on the photodiode as illustrated in FIG. 2b. The deflected image creates a signal that differs from the base-line signal. The changing signal from the photodiode results in a change in the output voltage of the refractometer. An integrator or chart recorder then registers the changes in output voltage as peaks on a chromatogram.
One factor that always creates issues in refractometry is temperature. The sample has to be maintained in a very thermally stable environment. Even slight changes or variations in temperature affect the density of the sample thereby changing its refractive index. Temperature also poses problems in elevated temperature polymer characterization and other high temperature analyses because the LED and the photodiode detector are electronic devices which generally cannot withstand very high temperatures.
It is generally known in the industry that taking one or both of the LED or the photodiode detector out of the high thermal environment is a solution to problems caused by high temperatures. However, taking both devices out of the high temperature thermal environment creates alignment, reliability and cost issues, and the refractometer is no longer a self contained unit. Taking one or both of the LED or photodiode detector out of the high thermal environment is usually accomplished by means of fiber optic cables that complicate the optical alignment. The fiber optic cables can degrade over time due to temperature and mechanical stress, causing reductions in light transmission. In addition to the cost of fiber optic cables, there are other costs associated with mounting parts at the fiber optic cable ends.
A known refractometer implementation aimed at allowing analyses at high temperatures is the Waters 150C Refractometer available from Waters Corporation, Milford, Mass. In this implementation, the light source, a tungsten lamp, is positioned outside of the high thermal environment while the photodiode detector is positioned within the high thermal environment to maintain the optical geometry of the system. As a result the Waters 150C Refractometer can only be used for analyses at 150.degree. C. or below or the integrity of the electronic photodiode detector will be compromised. Also, the signal to noise performance is generally negatively affected at temperatures above 100.degree. C. due to increased photodiode electrical noise at such elevated temperatures.